The present invention relates generally to induction motors of the multi-speed variety and more particularly, to methods for enhancing the efficiency of stator arrangements in a low speed mode without deleteriously affecting the efficiency in a high speed mode.
Multi-speed or pole changing motors are well known in the art. For example, induction motors have been designed having separate main windings with differing numbers of poles and separate start or auxiliary windings also having differing numbers of poles corresponding to the numbers of poles of the separate main windings. One main winding and one auxiliary winding are operational in one speed mode, and another main winding and another auxiliary winding are operational in a different speed mode. With winding arrangements of this type, there are, for a given speed mode, at least two windings which are not used, but which occupy stator core slot space thereby reducing the overall efficiency of the motor. Initial cost of the motor is also increased because the additional windings require additional slot space and additional winding material which is effectively unused in a given speed mode.
Other pole changing winding arrangements have been devised wherein a portion of the main and/or start winding is utilized in more than one speed mode. Thus, for example, a two speed motor may be devised by interconnecting the pole groups of a main winding and a start winding so that adjacent main poles or adjacent start poles are of dissimilar polarity for one speed of operation and those same adjacent pole groups are of a like polarity with induced opposite poles therebetween giving rise to a speed mode which is half the original speed. Other known arrangements include the use of a portion only of one or more of the windings. For example, the use of alternate ones only of the pole groups interconnected so that those alternate ones are of a differing magnetic polarity give rise to a mode of operation having effectively half the number of poles of the normal mode of operation for that winding. Such features may be employed for both a main winding and a starting or auxiliary winding, or for only one of such windings.
In still other known arrangements, frequently in conjunction with centrifugal or other start switching arrangements, multiple running windings are used in conjunction with a single start winding and the motor is always started in one speed configuration and then run on a selected one of the main or running windings.
Yet other known arrangements have met with particular success in configurations where the auxiliary winding continues to be energized during the running of the motor. Typical of these motor arrangements are the capacitor start, capacitor run circuit configurations. These configurations are often employed in conjunction with a switch such as, for example, a centrifugal switch, a current relay, or voltage sensing relay to change the effective capacitance of the auxiliary winding circuit. Such motors are frequently employed where significant starting torque is required such as, for example, in hermetic motor applications where the motor provides the driving force for a hermetically sealed compressor. One such known motor is a single phase, alternating current, pole changing motor which is operable in either of two speed modes. The motor includes a main winding having a plurality of pole groups disposed in selected stator core slots with the main winding operable in either speed mode to provide a magnetic field for motor operation. This motor may, for example, have two main winding pole groups connectable in a first configuration for two pole operation with two wound or actual magnetic poles, and connectable in a second configuration with two wound poles of like magnetic polarity and two consequent or induced poles for four pole operation. A first start winding for the lower speed mode of operation is shifted slightly from true quadrature with the corresponding poles created by the main winding because of power factor considerations and this first start winding lies radially innermost on the stator core. A second start or auxiliary winding for the higher speed mode of operation is disposed radially intermediate the first start winding and the main winding. The amount of wire which may be employed in the lower speed start winding in this known configuration is limited. Furthermore, a motor of this type frequently has a major portion of its operating time in the lower speed configuration and thus, it would be beneficial to increase the efficiency of such a motor in this lower speed range mode.